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1. Overcharge

The so-called overcharge is the process of continuing to charge after exceeding the specified charge termination voltage (usually 4.2V). In the case of overcharge, the battery capacity will be attenuated, mainly due to the following factors: ① the overcharge reaction of the graphite negative electrode; ② the positive electrode overcharge reaction; ③ the electrolyte oxidation reaction during overcharge. When the battery is overcharged, lithium ions are easily reduced and deposited on the surface of the negative electrode: Li＋＋e→Li(s)

The deposited lithium coats the surface of the negative electrode, blocking the insertion of lithium. The reasons for the decrease in discharge efficiency and capacity loss are: ①The amount of recyclable lithium is reduced; ②The deposited metallic lithium reacts with the solvent or supporting electrolyte to form Li2CO3, LiF or other products; ③The metallic lithium is usually formed between the negative electrode and the diaphragm, which may Blocking the pores of the separator increases the internal resistance of the battery. Fast charging, excessive current density, serious polarization of the negative electrode, and more obvious lithium deposition. The capacity loss caused by overcharge of the positive electrode is mainly due to the production of electrochemical inert substances (such as Co3O4, Mn2O3, etc.), which destroy the capacity balance between the electrodes, and the capacity loss is irreversible.

LiyCoO2→(1－y)/3[Co3O4＋O2(g)]＋yLiCoO2 y<0.4

At the same time, the oxygen generated by the decomposition of the positive electrode material in the sealed lithium ion battery does not have a recombination reaction (such as the generation of H2O) and the flammable gas generated by the decomposition of the electrolyte accumulates at the same time, and the consequences will be unimaginable. Overcharge will also cause the oxidation reaction of the electrolyte. The oxidation rate has a lot to do with the surface area of the positive electrode material, the current collector material and the added conductive agent (carbon black, etc.). At the same time, the type and surface area of the carbon black are also An important factor affecting the oxidation of electrolyte, the larger its surface area, the easier it is for the solvent to oxidize on the surface. When the pressure is higher than 4.5V, the electrolyte will oxidize to form insolubles (such as Li2Co3) and gases. These insolubles will block the micropores of the electrode and hinder the migration of lithium ions, resulting in capacity loss during the cycle.

2. Electrolyte decomposition

The electrolyte is composed of a solvent and a supporting electrolyte. After the positive electrode is decomposed, insoluble products such as Li2Co3 and LiF are usually formed. The battery capacity is reduced by blocking the pores of the electrode. The reduction reaction of the electrolyte will have an adverse effect on the battery capacity and cycle life. The gas generated by the reduction will increase the internal pressure of the battery, causing safety problems. The electrolyte is not stable on graphite and other lithium-intercalated carbon negative electrodes, and it is easy to react to produce irreversible capacity. The decomposition of the electrolyte during the initial charge and discharge will form a passivation film on the surface of the electrode. The passivation film can separate the electrolyte from the carbon negative electrode and prevent further decomposition of the electrolyte. Thereby maintaining the structural stability of the carbon negative electrode. Under ideal conditions, the reduction of the electrolyte is limited to the formation stage of the passivation film, and this process does not occur again when the cycle is stable. The reduction of the electrolyte salt participates in the formation of the passivation film, which is conducive to the stabilization of the passivation film, but the insolubles produced by the reduction will have an adverse effect on the solvent reduction products, and the concentration of the electrolyte decreases during the reduction of the electrolyte salt, which will eventually lead to Battery capacity loss (LiPF6 reduction produces LiF, LixPF5-x, PF3O and PF3). At the same time, the formation of the passivation film consumes lithium ions, which will cause the capacity imbalance between the two electrodes and reduce the specific capacity of the entire battery. The type of carbon used in the process, the composition of the electrolyte, and the additives in the electrode or electrolyte are all factors that affect the loss of film formation capacity. The electrolyte often contains substances such as oxygen, water and carbon dioxide. A small amount of water has no effect on the performance of the graphite electrode, but too high a water content will generate LiOH(s) and Li2O deposits, which are not conducive to lithium ion insertion and cause irreversible capacity loss: H2O＋e→OH－＋1/2H222 OH－＋Li＋→LiOH( s) LiOH＋Li＋＋e→Li2O(s)＋1/2H2

The CO2 in the solvent can be reduced to CO and LiCO3(s) on the negative electrode: 2CO2＋2e＋2Li＋→Li2CO3＋CO

CO will increase the internal pressure of the battery, while Li2CO3(s) will increase the internal resistance of the battery and affect the battery performance.

3. Self-discharge

Self-discharge refers to the natural loss of electric capacity when the battery is not in use. The capacity loss caused by self-discharge of lithium-ion battery is divided into two situations: one is reversible capacity loss; the other is irreversible capacity loss. Reversible capacity loss means that the lost capacity can be restored during charging, while the irreversible capacity loss is the opposite. For example, the lithium manganese oxide positive electrode and the solvent will interact with the microbattery to produce self-discharge and cause irreversible capacity loss. The degree of self-discharge is affected by factors such as the cathode material, the manufacturing process of the battery, the nature of the electrolyte, temperature and time. For example, the self-discharge rate is mainly controlled by the oxidation rate of the solvent, so the stability of the solvent affects the storage life of the battery. If the negative electrode is in a fully charged state and the positive electrode self-discharges, the battery content balance will be destroyed, which will result in permanent capacity loss. During prolonged or frequent self-discharge, lithium may be deposited on the carbon, increasing the capacity imbalance between the two stages. Pistoia et al. believed that the oxidation product of self-discharge blocks the micropores on the electrode material, which makes the insertion and extraction of lithium difficult, increases the internal resistance and reduces the discharge efficiency, resulting in irreversible capacity loss.

4. Electrode instability

As described above, the positive electrode active material will oxidize and decompose the electrolyte in the charged state, causing capacity loss. In addition, the factors affecting the dissolution of the positive electrode material are the structural defects of the positive electrode active material, the excessively high charging potential and the content of carbon black in the positive electrode material. Among them, the most important factor is the electrode structure change potential during the charge and discharge cycle.

Lithium cobalt oxide is a hexagonal crystal in a fully charged state, and a new phase monoclinic crystal is formed after 50% of the theoretical capacity is discharged. The lithium nickel oxide involves rhombohedral and monoclinic changes during the charge-discharge cycle. LiyNiO2 is usually Cycle within the range of 0.3<Y<="">. There are two different structural changes in lithium manganese oxide during the charge and discharge process: one is the phase change that occurs when the stoichiometry is unchanged; the other is the phase change that occurs when the amount of lithium insertion and extraction changes during the charge and discharge process. When the charging voltage of LiCoO2 lithium ion battery exceeds 4.2V, the capacity loss is directly related to the cobalt content detected in the negative electrode, and the higher the charge cut-off current voltage, the greater the rate of cobalt dissolution. In addition, the capacity loss (or cobalt dissolution) is related to the heat treatment temperature of the synthetic active material.

5. Current collector

Copper and aluminum are the most commonly used materials for negative and positive current collectors, respectively. Among them, aluminum foil is easier to form an oxide film on the surface regardless of whether it is in the air or in the electrolyte. At the same time, the overall corrosion and local corrosion (such as pitting) and poor adhesion of the current collector surface will increase the electrode reaction resistance. , The internal resistance of the battery increases, resulting in a loss of capacity and a decrease in discharge efficiency. In order to reduce the influence caused by these reasons, the current collectors purchased from the market are best to be pretreated (acid-alkali erosion, corrosion-resistant coating, conductive coating, etc.) to improve corrosion resistance and adhesion performance. Because the surface adhesion of the current collector is too small, the electrode may be partially separated from the current collector, which increases the polarization and has a great influence on the capacity. The copper current collector corrodes during use to form a layer of insulating corrosion product film. As a result, the internal resistance of the battery increases and the discharge efficiency decreases during the cycle, resulting in a loss of capacity. When over-discharged, the copper foil will react as follows:

Cu → Cu + + e- produced Cu (I)

When charging, it will crystallize and deposit in the form of metallic copper on the surface of the negative electrode, forming copper dendrites, which can easily penetrate the diaphragm and cause a short circuit or even an explosion. Special attention should be paid to the selection of negative pole pieces. It is absolutely not allowed to have pole pieces that are exposed to copper, otherwise it is easy to generate dendrites at the exposed copper to damage the battery. To prevent the dissolution of the copper current collector, the discharge voltage should not be lower than 2.5V.